Stress Analysis Project
Jennifer Elkin, Julie Rekant, Jacqueline Shaw
The Creation: The Short Version of the Long Story
The structure shown above has been through many upgrades over a series of three design reviews. Back on February 21, 2012, it was more of an artistic collage of aluminum pieces rather than a supportive weight bearer. As expected, this original design failed miserably because the arm bent with any kind of weight that came into contact with it. However, for the second design review on March 27, 2012, the design was completely remodeled. It started once again from the drawing board, and turned into an almost-working model. The only problems with it were that it only lifted the weight about one to one and a half inches, and the base was too big by about a half of an inch to an inch. For the third design review, finalizing tweaks were made to perfect it. The final design was very sturdy, and it successfully lifted the weight two inches.
How It Works
This mechanism is designed to clamp into two parallel clamps at the base. The base is a three-story hollow rectangular prism with strips of aluminum in tension for support. The arm bends and goes through a large hole all the way to the weight on the other side of the course. The arm is attached to the side of the middle story of the base, and consists of two T-beams located on the top and bottom. In the middle of the arm, there are supporting vertical aluminum strips along with diagonal ones in tension. The motor is mounted onto the end of the arm by small aluminum pieces and that are screwed onto the structure in a way that does not allow the motor to move. A thick black component is attached to the motor, and rotates along with the weight. The L-beam between the black piece and another aluminum strip has a small diagonal aluminum strip attached that grabs onto the screw from the weight and lifts that screw as the motor rotates. The counterweight attached to the other end of the black piece creates a larger force causing the black piece to turn. With all of the screws fastened as securely as possible, the structure works exactly as it is supposed to work.
According to the manufacturer information, the servo is able to produce a torque of up to 72 oz-in, and has the capacity to rotate 90 degrees. This specific design creates an upwards force on the weight due to the normal force from the L-beam, which is attached to the servo. The normal force adds a downward force on the arm, which produces a moment of the arm about the servo that the counterweight on the arm has to balance. Along with the moment imparted by the servo, the counterweight creates a moment by concentrating a weight force far away from the servo. These moments worked together against the moment imparted by the one-pound weight so that the arm rotates in the desired direction. The counterweight’s center of weight is approximately ten inches from the weight. The weight of the counterweight is approximately one pound within a margin of error of two or three ounces. Assuming it is one pound, the torque it supplies to the weight is (16 ounces)*(10 inches) = 160 oz-in. This, on top of the concentrated torque from the motor of 72 oz-in, creates a torque of about 232 oz-in. The black component arm is about a foot long, so the weight places an opposing couple on the end of the black component equal to (16 ounces)*(12 inches) = 192 oz-in. This difference in torques creates a net couple with the weight moving up two inches. The net moment is 40 oz-in roughly. In theory, the system is able to lift 2.5 inches, but there were losses due to both deflection (which we are very proud to say was extremely slight) and other things like heat energy. To lift 16 ounces two inches, a net torque of about 32 oz-in is necessary, which is just below the calculated value of 40 oz-in in which the system was theoretically able to supply.
This design is unique compared to other designs. First of all, the two T-beams in the arm add to the stiffness of the arm. This is a proud point in our design because it marked the major difference between our original artistic collage and our final functional structure. Another notable feature is the L-beam at the end that lifts the weight. The L-beam is attached to a diagonal aluminum strip that eases the ability of the structure to grab onto the weight. This is a proud point because one of our hardest challenges was being able to grab the weight in order to lift it. This L-beam plus that small aluminum piece finally put an end to that problem that we faced.